The f-number is the focal length divided by the diameter of the entrance pupil. So for the same f-number, a longer lens needs a much larger effective aperture. A 105mm lens at f/1.2 would need a dramatically larger opening than a 50mm at f/1.2.

But size alone isn’t the whole story. Making a lens faster increases optical aberrations such as spherical aberration and coma, so the design becomes much harder to correct while keeping good image quality. Larger elements also require much more glass, making the lens bigger, heavier, and more expensive.

There are also practical limits: the camera mount can constrain how large the rear opening can be, and manufacturers only build extremely fast lenses when the demand justifies the cost and complexity.

Macro lenses add another design priority: they are optimized for close-focus performance and flat-field sharpness, which often leads to more modest maximum apertures. So a 105mm macro could be made faster in principle, but it would likely be much larger, heavier, costlier, and harder to correct optically.